A process for the clarification treatment of municipal sewage or industrial wastewater, potable water, process water or wastewater, which employs a ballasted flocculation technique. The treatment consists of injecting, into the water to be treated, a contacting medium (consisting of a granular material, such as sand) and a polymer and then in carrying out a flocculation step which is accelerated by this contacting medium and this polymer. This is followed by a settling step. The sand, after it has been separated from the sludge and cleaned, is recirculated into the flocculation step. An example of how such a process is employed is described in particular in FR-A-1,411,792.
This known process has effectively the advantage of accelerating the flocculation, thereby making it possible to achieve a relatively large reduction in the residence time of the water to be treated, in the flocculation phase, of about 2 to 8 minutes. Moreover, this process makes it possible to achieve very high settling velocities (greater than 15 m/h), these velocities possibly being as high as and even exceeding 100 meters/hour when accelerated settling with lamellar modules is employed. In addition, the plants can be started up quickly (in approximately ten minutes) because of the immediate availability of the granular material in the settling unit.
However, experience shows that this process has a number of drawbacks which are summarized below:
1. High Electrical Consumption
The sand/sludge separation which takes place using a hydrocyclone consumes a great deal of energy. Furthermore, the operation of the hydrocyclone cannot be adjusted according to the flow rate or the quality of the raw water. This is because the hydrocyclone cannot be shut down, since taking it out of service would cancel out the flocculation and the settling; in addition, the hydrocyclone-type systems for cleaning the granular material must, in order to ensure stable flocculation and simply because of their principle, be used with a fixed throughput, with a continuous influx of polymer and granular material: they are therefore sized to the maximum throughput and to the maximum material loading of the settling station. As a result, the electrical consumption, per m.sup.3 of treated liquid, is obviously higher for the low throughputs and for water with a low content of suspended matter.
2. Management of Plant Shut-downs
Moreover, the systems described at the present time mean that, during shut-down periods, the granular material does not reside at the bottom of the settler but is transferred into the flocculator: there is in fact a high risk, when restarting the plant, of blocking the scraper of this settler (if the granular material remains on the floor) or the hydrocyclone (blockage of the hydrocyclone and/or loss of sand at the overflow are inevitable when the concentration of granular material in the draw-off hopper is high). It is then necessary to transfer the sand either to the flocculator, the propeller, which has to be oversized in order to ensure that the sand is resuspended, or to a specific tank having high-energy stirring means.
3. Pipe Blockage
In conventional systems (especially FR-A-2,719,234), it is also necessary, in order to avoid as far as possible blockages of the draw-off circuit, to position the pump or pumps for drawing off the ballasted floc as close as possible to the central draw-off region of the settler. This implies the construction of a service duct below the floor of the said settler and the fitting of long horizontal and vertical pipes, which are always sensitive to blocking. These requirements considerably increase the construction and running costs of the conventional plants.
4. Low Sludge Concentration
The very principle of the hydrocyclone entails a high loss of water (which may amount to 3 to 20% of the throughput actually treated in the plant) and, as a corollary, the production of diluted sludge at the overflow of the said hydrocyclone; the sludge dilution is greater the smaller the throughput treated, or the lower the content of suspended matter in the raw water. The concentration obtained is about 0.1 to 2 grams per liter in the case of the clarification of surface water.
On account of the drawbacks of this prior art, the objectives of the present invention were especially to solve the following problems:
to decrease the energy consumption, particularly for low throughput or in the case of a low content of suspended matter in the influent liquid; PA1 to manage the shut-down and restart periods of the settler and to eliminate the risks of blockage; PA1 to limit the loss of water with respect to the total throughput treated by the plant and to obtain sludge having a high concentration (for example, greater than 2 grams per liter in the case of the clarification of surface water); PA1 to reduce the set-up and running costs by dispensing with the pumps (and therefore the services ducts) located beneath the floor of the settlers and to simplify the running of the plants employing the technique of ballasted flocculation. PA1 "ballast": the granular material (for example sand), either fresh or recycled after cleaning the ballasted floc; during the first operation of the plant, all the ballast is introduced into the "flocculator-settler" equipment and, subsequently, the injection of fresh ballast will only be exceptional, for example to compensate for losses that are liable to occur or as a result of accidents); PA1 "ballasted floc": the agglomerate consisting of the ballast, the polymer and the particles which have to be removed from the liquid during clarification; PA1 "sludge": the residue obtained after the ballasted floc has been cleaned (and after the clean ballast has been recovered), for example at the overflow of the hydrocyclone; PA1 "ballast ratio": the proportion, expressed as a percentage, clean ballast/(total ballast+sludge), measured in the flocculator; a value of 90% corresponds to a "dirty" ballast, laden with sludge to an amount of 10% of its weight, whereas a ballast ratio of 99% (and higher) corresponds to a clean ballast. PA1 a high-energy regime, ensuring draw-off of the ballast contained in the ballasted floc (and therefore, for example, separation of the sand from the sludge), removal of the floc in the form of sludge and recycling of the cleaned ballast into the flocculation unit and, PA1 a low-energy regime ensuring transfer of the ballasted floc into the flocculation unit as long as the ballast ratio in the flocculator is greater than a predetermined level, this low-energy regime remaining in service while the high-energy regime is being put into action, but then being shut down during the operation of the high-energy regime, the two systems being employed alternately so as to maintain the ballast ratio at a predetermined level. PA1 a low-energy system consisting of a recirculation tube placed in the central region for taking up the ballasted floc in the settler which can be of conical or cylindroconical shape or have a flat and scraped bottom, one of the ends of the tube being positioned in the immediate vicinity of the bottom of the settler, and of a means for pumping the ballasted floc through the tube in order to deliver it to the flocculator when the ballast ratio in the latter is above the said predetermined level, and PA1 a high-energy system which comprises, for example, a hydrocyclone and a suction pipe feeding the hydrocyclone, drawing off the ballasted floc at the bottom of the settler, in the same region as the recirculation tube of the low-energy system, the hydrocyclone ensuring, as is known, separation of the ballast with the sludge and its recycling to the flocculator.
In the description which follows, the following terminology has been adopted: